The present invention relates to optical communication systems, and more particularly to an optical source having reduced relative intensity noise (RIN).
An important aspect of a communication system is its ability to distinguish the signal from the noise and thereby yield a distortion free and error free reproduction of the original signal. Thermal noise results from the agitations of molecules within a system, which typically increase with temperature. In a simple resistor, random and erratic wanderings of the electrons within the resistor structure will cause statistical fluctuations away from electrical neutrality. Thus, at one time or another the distribution of charge may not be uniform, causing a voltage difference to appear between the resistor terminals. The random, erratic, unpredictable voltage which results is referred to as thermal resistor noise.
A second type of noise results from a phenomenon associated with the flow of current across semiconductor junctions. The charge carriers, electrons or holes, enter the junction region from one side, drift or are accelerated across the junction, and are collected on the other side. Although the average junction current determines the average interval that elapses between the times when two successive carriers enter the junction, the exact interval that elapses is subject to random statistical fluctuations. This randomness gives rise to a type of noise that is commonly referred to as shot noise.
Optical communication systems are not immune to such noise phenomena. For example, the problem of photon noise is discussed in P. R. Morkel, R. I. Laming, H. O. Edwards and D. N. Payne, "Elimination of Excess Photon Noise from Fiber Superluminescent Sources," Paper CTuH76 presented at the CLEO Conference in Anaheim, Calif. on May 22, 1990. It is well known that the relative intensity noise of a superluminescent source is high. Most lasers used in optical communication systems also suffer from a relatively high level of RIN.
Higher RIN levels increase the noise floor of an optical carrier generated by a light source. The noise floor of an optical carrier is advantageously as low as possible. For example, an RIN on the order of -125 dB/Hz is generally acceptable. Once the RIN rises to above -100 dB/Hz or so, the signal quality will suffer appreciably.
It would be advantageous to provide apparatus for producing an optical carrier with low relative intensity noise. Such apparatus should be easy to implement and cost effective. It would be further advantageous to provide such apparatus that is successful in reducing the RIN level of various types of light sources, including superluminescent sources and lasers.
The present invention provides apparatus having the aforementioned advantages.